report by Lil Squid from the Noun Project. See:https://thenounproject.com/term/report/149914

Waswa, P. M., & Redkar, S. (2019, August). Lucid Analysis of Periodically Forced Nonlinear Systems via Normal Forms. In International Design Engineering Technical Conferences and Computers and Information in Engineering Conference (Vol. 59261, p. V006T09A044). American Society of Mechanical Engineers.